A photodiode is a type of photodetector capable of converting light into either current or voltage, depending on the mode of operation. More specifically, a photodiode is a photosensitive semiconductor diode typically having either a p-n junction or a PIN structure. When a photon having sufficient energy is incident on the photodiode it is absorbed and an electron-hole pair is generated. When an electric field is applied across the photodiode, the electron-hole pairs are swept away, thus inducing a photocurrent.
Most photodiodes are designed to be used in either the photoconductive mode (reverse biased) or the photovoltaic mode (no bias). In the photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. A current flows when the device is loaded. In the photoconductive mode, the drift of carriers induces a reverse current dependent on the incident optical power, which is delivered to an outer circuit.
In general, the total current passing through the photodiode is the sum of the photocurrent (i.e., the electrical current that flows through the photodiode as a result of the exposure to the photons having sufficient energy) and the dark current (i.e., the electrical current that flows through the photodiode even when it is not exposed to the photons having sufficient energy). In fact, dark current, which is a property of all photodiodes and is due to the random generation of electrons and holes within the depletion region of the photodiode, is one of the main sources of noise in devices using photodiodes. Accordingly, it is advantageous to minimize dark current in order to maximize the sensitivity of these devices.
In general, dark current is primarily a function of the semiconductor material, the size (e.g., active area) of the photodiode, the temperature of the photodiode, and the bias voltage across the photodiode. For example, dark current typically increases with increases in temperature. Accordingly, one method of reducing dark current in devices having photodiodes is to cool the photodiode. For example, some high sensitivity optical power meters, with sensitivities below −80 dBm, mount a thermo-electric cooler (TEC) directly under the photodiode substrate in order to stabilize the dark current present in the photodiode.
FIG. 1 illustrates one embodiment of an optical power meter (OPM) having a TEC cooled photodiode. The OPM, which is a device commonly used to measure the power in an optical signal in a fiber optic system, includes the TEC cooled photodiode 110, an amplifier 120, an analog-to-digital converter (ADC) 130, a controller 140, and a display 180. The photodiode 110 is formed of a semiconductor material selected for the particular application. For example, semiconductor materials such as silicon (Si), Germanium (Ge), and/or Indium-Gallium-Arsenide (InGaAs) are commonly used in OPMs since they are sensitive to light at the wavelengths and power levels common to fiber optics. When the photodiode 110 is exposed to light at the appropriate wavelength, a photocurrent is generated, which is dependent on the power of the incident light. The total current passing through the photodiode, including the photocurrent, is amplified by and converted to a voltage by the amplifier 120. The resultant voltage is converted to a digital signal by the analog-to-digital converter (ADC) 130. The controller 140 processes the digital signal to generate a result (e.g., an optical power reading) and sends the result to the display 180. Notably, although dark current is significantly reduced by TEC cooling, it is still present. Accordingly, an initial dark current measurement wherein the optical input to the OPM is blocked is typically obtained and stored. The controller 140 then uses this initial dark current reading to calculate more accurate readings for subsequent optical power measurements (e.g., subsequent power measurements are computed after subtracting the initial dark current from the total photodiode current).